The desirability of providing an intake manifold for an internal combustion engine having a variable flow length has been recognized previously. Selection of a specific flow length for the intake manifold generates a specific resonant frequency at a specific engine speed. For this specific engine speed, the volumetric efficiency or flow is maximized, In the past, relatively long ram tube type manifolds have developed which are very effective at a given engine speed, but which may only provide advantageous operation over a relatively narrow speed range. Accordingly, it is desirable to provide a manifold with a selectively variable flow length.
Under relatively low engine speed operating conditions, the frequency of the engine inlet valve opening cycle is relatively low. Thus, the frequency or pulsations of the air column within the manifold is also low and the time between pulsations is relatively long. Accordingly, under low speed engine operation, a relatively long manifold flow length is desirable in order to achieve resonance and, hence, a desirable high efficiency engine breathing. However, under relatively high engine speed operating conditions, the frequency of the engine inlet valve opening cycle is relatively high and, also, the frequency of air column pulsations. Under these conditions, the time between such pulsations is rather short. Thus, a relatively short air flow length is desirable to achieve a resonant condition and resultant high efficiency breathing for the engine.
The U.S. Pat. No. 3,370,575 discloses a dual flow path manifold for an engine. Controls are provided to alternately shift the flow from one flow path to a second flow path.
U.S. Pat. No. 1,598,046 discloses a manifold for an internal combustion engine provides two alternate flow paths, each having a separate carburetor.
U.S. Pat. No. 2,894,497 discloses a manifold with an effective variable length air passage controlled by an engine governor device. The variable length portions of the device include telescoped metal tubes with portions of the surface removed so that, as the tubes are rotated relative to one another, the effective air flow is varied.
U.S. Pat. No. 4,543,918 discloses an intake manifold with a plurality of tubular air flow sections within a plenum arranged serially so that the sections may be combined end to end with without flow gaps therebetween, thus effectively providing a desired tuned air flow.